Metallized plastics are commonly used for decorative purposes. For instance, the surfaces of plastics, such as acrylonitrile-butadiene-styrene (ABS) and ABS-Polycarbonate blends, are metallized for use in sanitary fittings, automobile accessories, furniture, hardware, jeweries, and buttons/knobs. These articles of manufacture may be metallized to impart an attractive appearance to the article surfaces.
In addition, plastics, rubbers, and polymer matrix composites (e.g. fiber-reinforced or additive-filled thermoplastic, thermoset, and rubber matrix composites) can also be metallized for functional purposes. For instance, metallization of plastics-based electronic components may be carried out for the purpose of shielding against electromagnetic interference (EMI). Additionally, the surface properties of polymeric components can be altered in a controlled manner through metallic coating.
Articles made from an electrically nonconductive polymer (e.g., plastic, rubber, polymer matrix composite, etc.) can be metallized by an electroless metallization process. In a typical process, the article is first cleaned and etched, then treated with a noble metal (e.g. palladium) and finally metallized in a metalizing solution. The etching step typically involves the use of chromic acid or chromosulfuric acid. The etching step serves to make the surface of the article receptive to the subsequent metallization through improved surface wettability by the respective solutions in the subsequent treatment steps and to make the ultimately deposited metal being well-adhered to the polymer surface.
In the etching step, the surface of a polymer article is etched using chromosulfuric acid to form surface micro-caverns in which metal is deposited and adhered. After the etching step, the polymer component surface is activated by means of an activating agent (or activator), typically comprising a noble metal, and then metallized using electroless plating. Subsequently, a thicker metal layer can be deposited electrolytically.
Chromosulfuric acid-based etching solutions are toxic and should therefore be replaced where possible. For instance, the etching solutions based on chromosulfuric acid may be replaced with those comprising permanganate salts. The use of permanganates in an alkaline medium for metallization of circuit boards as a carrier of electronic circuits has long been established. Since the hexavalent state (manganate) which arises in the oxidation is water-soluble and has sufficient stability under alkaline conditions, the manganate, similarly to trivalent chromium, can be oxidized electrolytically back to the original oxidizing agent, in this case the permanganate. For the metallization of ABS plastics, a solution of alkaline permanganate has been found to be unsuitable since it was not possible in this way to obtain a sufficient adhesion strength between the metal layer and plastic substrate. This adhesion strength is determined in the “peel test” and should have at least a value of 0.4 N/mm.
As an alternative to chromosulfuric acid, WO 2009/023628 A2 proposes the use of strongly acidic solutions comprising an alkali metal permanganate salt. The solution contains about 20 g/l alkali metal permanganate salt in 40-85% by weight phosphoric acid. Such solutions form colloidal manganese(IV) species which are difficult to remove. Further, it is also difficult for colloids to form a coating of adequate quality. To solve the problem, WO 2009/023628 A2 proposes the use of manganese(VII) sources which do not contain any alkali metal or alkaline earth metal ions. However, the preparation of such manganese(VII) sources is costly and inconvenient.
Thus, there is an urgent need to conduct industrial scale metallization of polymer component surfaces without using chromic acid, chromosulfuric acid or an alkali metal permanganate salt.
Another major issue of the prior art metallization process is the notion that, after the etching step, the polymer component surface must be activated by means of an activating agent, which typically comprises a noble metal (e.g. palladium). The noble metals are known to be rare and expensive. In an alternative process [L. Naruskevicius, et al. “Process for metallizing a plastic surface,” U.S. Pat. No. 6,712,948 (Mar. 30, 2004)], the chemically etched plastic surface is treated with a metal salt solution, containing cobalt salt, silver salt, tin salt, or lead salt. However, the activated plastic surface must be further treated with a sulfide solution. The entire process is slow, tedious, and expensive.
Thus, there is a further urgent need to conduct industrial scale metallization of polymer component surfaces without using an expensive noble metal in an activating agent or even without the activating step if all possible.